The system used for casting electrodes with integral lifting lugs for metal electro-deposition processes traditionally includes a number of molds placed at the perimeter of a casting wheel. The wheel either advances the molds one position and stops for filling of the mold and for removing the cooled electrode, or rotates at a steady speed and filling and removing occur while the wheel is in motion.
This system has a number of serious disadvantages. Variations in thickness in the lifting lugs of the electrodes make it difficult for the grabs of a cellhouse crane to pick up a group of electrodes from an electrolytic cell. Because of thickness variations, an operator is required to manipulate the grabs over the electrodes until every electrode in the group is securely held in the grabs before lifting. To use a crane effectively, electrodes with a minimum variation in thickness should be used.
A second disadvantage is the occurrence of a flash of metal at the edges of the electrodes. Flash is a source of electrical shorts during electro-deposition and must be accommodated in the cells by providing sufficient, i.e. increased, spacing between electrodes.
Another reason for increased spacing between electrodes in a cell is the usual provision of a thicker section in the upper portion of an electrode between the lifting lugs. This thicker section is necessary to ensure that there is sufficient metal left after the electro-deposition process to prevent buckling between the lifting lugs in either the grab means of a crane or in other equipment. If an electrode buckles it drops away from the grab means and falls, causing a serious safety hazard and/or damage to equipment.
Variations in lifting lug thickness and flash result from the way melt is poured into the mold. The pouring causes waves to be set up in the molten metal. Because of the "throttling" effect of the lifting lug shape, these waves are amplified in the lifting lugs. If a crest of the wave impinges on the mold walls in the lifting lugs at the moment that the metal solidifies, the metal freezes to the thickness of the original molten wave crest. Whereas another time it may be a valley in the wave that freezes when it impinges on the mold wall. This wave phenomenon, therefore, causes the thickness of the lifting lugs to vary considerably.
In order for the molten metal to solidify the mold is kept at a temperature considerably lower than the freezing point of the metal. Waves created in the main body of an electrode hit the side wall and climb up onto it. Because the side wall is at a lower temperature, the skin of the wave crest in contact with the mold freezes while the remainder of the wave drops back. This results in projections (flash) along the edges of the electrode.
Electrodes with even thickness could be cast in book molds. Although book molds can be used for making copper bullion electrodes for use in a copper refinery, book molds are not suitable for casting electrodes of lead bullion for use in a refinery for the electro refining of lead by the Betts or the bipolar process. Experiments with book mold-cast lead bullion electrodes have shown that the resultant varying fine and coarse grain structures did not provide sufficient strength, causing normally adhering slime to fall into the cells.